Choose the missing terms out of the given alternatives : AI, BJ, CK, ?

The answer is $\boxed{\text{DL}}$.

The sequence is formed by adding 3 to the previous letter.

• A + 3 = B
• B + 3 = C
• C + 3 = D

Therefore, the next letter in the sequence is $\boxed{\text{DL}}$.

Option A is incorrect because $D$ is not the next letter in the sequence.

Option B is incorrect because $DM$ is not the next letter in the sequence.

Option C is incorrect because $GH$ is not the next letter in the sequence.

Option D is correct because $DL$ is the next letter in the sequence.